The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Wideband VCOs are electronic oscillators where the oscillation frequency may be tuned over a large range of frequencies. In one design approach, the wideband VCO's large tuning range is realized by using a switched capacitor array that is electronically coupled in parallel with a single varactor connection and an inductor. The switched capacitor array enables coarse frequency tuning via a digital control signal that switches capacitors into and out of the LC tank. Such coarse tuning is typically referred to as digital tuning since different digital control codes may be used to switch between different operating frequency bands. Within each frequency band, fine tuning, also referred to as analog tuning, is realized by applying a control voltage to the varactor to adjust the varactor's capacitance.
While the above approach allows for a large tuning range, the cube of the oscillation frequency is directly proportional to the VCO's gain, represented as KVCO, when only one varactor is used. Due to this relationship and the nonlinear capacitance-to-voltage characteristic response of the varactor, KVCO varies widely both over different control voltages within a given frequency band and across different frequency bands supported by the switched capacitor array. Generally, the KVCO variation increases as the frequency tuning range of the VCO is stepped up.
A large KVCO variation has negative implications for phased locked loops (PLLs) in which such wideband VCOs are employed. For example, a change in KVCO may cause the PLL loop bandwidth to vary between different frequency bands, increasing phase noise at higher frequencies. In addition, the loop stability of the PLL may be negatively impacted, resulting in longer settling times, phase error, and/or jitter. Thus, large KVCO fluctuations may significantly degrade the performance of such electronic circuits.